The present invention relates to a control system for controlling a synchronous motor to maintain a constant torque, and more particularly to a synchronous motor control system capable of improving torque characteristics of a synchronous motor at high speeds and on acceleration.
Servomotors have found use in a variety of applications. In recent years, there has been developed an AC servomotor, so that a synchronous motor is available as a servomotor. Since a synchronous motor having a permanent magnet used as a rotor is of the brushless type, the motor generates no noise, is simple in construction, and hence is in wide use.
The synchronous motor is required to be controlled to produce a constant torque. To meet this requirement, a technique has been developed for passing a current in phase with an electromotive force induced by a rotor, through an armature winding serving as a stator. Such a technique will be described with reference to FIG. 1 which shows the arrangement of a synchronous motor. A flux density B in a position spaced at an angle .theta. from a q-axis of the magnetic field of a rotor 1 which is composed of a permanent magnet is given by: EQU B=Bm.multidot.sin .theta. (1)
A magnetic flux .phi. crossing an a-winding of stator 2 is expressed as follows: EQU .phi.=-.phi.m.multidot.cos .theta.c (2)
(where .phi.m indicates a magnetic flux on the q-axis of the rotor 1.)
Therefore, an electromotive force e.sub.1 induced across the a-winding is given by: EQU e.sub.1 =-(d.phi./dt)=-.phi.m.multidot.p.multidot..omega.m.multidot.sin .theta. (3)
(where .theta.=P.theta.m=P.multidot..omega.m.multidot.t.)
Likewise, electromotive forces e.sub.2, e.sub.3 induced across b- and c-windings of the stator 2 which are disposed in positions spaced 1/3.pi., 2/3.pi., respectively, from the a-winding are expressed by: EQU e.sub.2 =-.phi.m.multidot.P.multidot..omega.m sin (.theta.+2/(3.pi.)) (4) EQU e.sub.3 =-.phi.m.multidot.P.multidot..omega.m sin (74 +4/(3.pi.)) (5)
Let currents flowing through the armature windings a, b, and c be expressed by i.sub.1, i.sub.2, i.sub.3, and an output torque T of the three-phase synchronous motor is expressed as follows: EQU T=1/2(e.sub.1 .multidot.i.sub.1 +e.sub.2 .multidot.i.sub.2 +e.sub.3 .multidot.i.sub.3) (6)
By substituting the equations (3), (4), and (5) for the electromotive forces in the equation (6), ##EQU1## For making the torque T constant, it should not be dependent on the angle .theta.. By selecting the currents: EQU i.sub.1 =I sin .theta. EQU i.sub.2 =I sin (.theta.+2/(3.pi.)) (8) EQU i.sub.2 =I sin (.theta.+2/(3.pi.))
(where I is the amplitude of the current), the torque T of the equation (7) becomes ##EQU2## Therefore, the torque T is constant regardless of the rotational position of the rotor 1.
For effecting such control, it is necessary to detect the angular position of the rotor of the synchronous motor for thereby controlling the value of each armature current.
A drive control device for such a motor includes an inverter at a final stage. Since an inverter has a physical saturable quantity (highest voltage), when a current command in excess of the saturable quantity of the inverter is given (e.g., at the time of operation at a high speed or during acceleration), a current waveform supplied to the synchronous motor is no longer a sine wave. More specifically, as shown in FIGS. 2(A), (B), and (C), the inverter is saturated at a maximum voltage .vertline.V.vertline., and the current waveform in each of R, S, and T phases approaches a rectangular wave.
On high-speed operation or acceleration, therefore, the constant K in the equation (9) is lowered to reduce the torque efficiency, with the results that no effective torque is produced and smooth rotation is impaired, resulting in noise and vibrations.
To prevent the current waveform supplied to the synchronous motor from reaching the rectangular wave, it would be possible to multiply an effective current command by k (0&lt;k&lt;1). With such a system, however, it would multiplied by sin .theta., and the difference between the product and a current value would be determined and issued as a current command to the inverter. Therefore, it would be difficult to ensure accurately that the output to the inverter would be within the saturable quantity. Alternatively, if the constant k were small, the synchronous motor would fail to produce a sufficient torque.